Human Heart, Eels Generate Interest as Implant Power Sources

When attached to a pacing lead, a microgenerator can provide one-third of the electricity needed to operate a pacemaker.

The industrywide trend toward miniaturization is presenting device makers with what seems like a paradoxical request: Reduce the size of implantable devices while increasing functionality. Adding functionality, however, typically translates into increasing the size of the battery, and thus the overall package size, to power these new features. Because of this design conundrum, researchers have shifted their focus to augmenting or even eliminating batteries as implant power sources. Instead, researchers are exploring more-natural power sources such as the human heart and electric eels to support next-generation implantable devices.

One such project involves a UK-based consortium that has designed and clinically tested an in-body microgenerator able to harvest energy from a human heartbeat to power such implants as pacemakers and implanted cardio defibrillators. Lead by Zarlink Semiconductor (Ottawa, ON, Canada; www.zarlink.com), the Self-Energizing Implantable Medical Microsystem (SIMM) project consists of several UK companies in collaboration with clinicians at Southampton University Hospital (Southampton, UK; www.suht.nhs.uk).

"The driver for us was to allow more functionality to be incorporated into the pacemaker by making the battery smaller and being able to drive more power into the pacemaker to power extra functions and therapies," says Martin McHugh, business development manager for Zarlink's Advanced Packaging group and SIMM project coordinator. The consortium sought to achieve this goal by harvesting the differential energy in the chambers of the human heart to drive a linear generator. By augmenting the battery using the natural in-body energy supply, the microgenerator was able to provide one-third of the electricity needed to operate a pacemaker, according to the group.

The design of the microgenerator was strongly influenced by the team's desire to incorporate the part into the existing device assembly. By not altering the design of the pacemaker, the consortium hopes to avoid delays in market launch and to enable a seamless transition for doctors. "If the surgeon was implanting a pacemaker and the next day a microgenerator was implemented, he shouldn't technically see any difference in his procedure," McHugh says. The group achieved this design goal by integrating the microgenerator into the pacing lead.

Another driver of the microgenerator's design proved to be the requirements for harvesting energy. Other systems have necessitated that a person perform some sort of voluntary physical activity, such as walking, in order to generate energy, according to McHugh. In contrast, he adds, the SIMM microgenerator can produce energy through both voluntary and involuntary actions, such as sleeping.

Once optimized, incorporation of the microgenerator could allow for smaller implant batteries in the packaging, thereby allowing for more space inside the device for additional components. Freeing up internal real estate in the device opens the door for integration of added functionality in order to enhance patient care. McHugh cites increased wireless technologies--Zarlink's specialty--for home monitoring capabilities as well as various sensors and components aiding in therapeutic tasks as potential candidates to fill the newly available space.

Although the first iteration of the microgenerator will likely augment smaller batteries, future versions could replace batteries entirely. The consortium predicts that the first models of the microgenerator could be ready for general use in as little as three years.

Flashing forward 10 years into the future, some implantable devices could be powered via the same mechanisms used by the electric eel to shock its predators and prey.

While exploring the use of nanotechnology for biological energy conversion systems, researchers at Yale University (New Haven, CT; www.yale.edu) and the National Institute of Standards and Technology (NIST; Gaithersburg, MD; www.nist.gov) sought to understand how eels produce their electric shock. The researchers found that an eel's cells generate electric pulses from several ion channels and pumps that act as natural nanoscale conductors. Employing computer models, the researchers labored to engineer artificial cells that functioned in the same manner.

"In the simplest terms, the electrocyte [electrogenic cell] converts chemical energy from food into an ion concentration gradient, which is a means to store energy," explains David LaVan, a NIST researcher. "The electrocyte then releases the ion concentration gradient in pulses that convert the chemical potential energy into electricity. We showed that many phenomena, such as the relationship between energy delivered to trigger the cell and the resting time before the cell can fire again are related to the ion channels and ion pumps and do not need to be explicitly defined."

Upon successfully creating these models, the team discovered that the artifical cells were quite efficient. The natural electrocytes boast an efficiency of roughly 15%, which LaVan believes could be further improved through design modifications.

Armed with a strong model, the researchers' next step is taking their findings from the computer to the lab. "With a fundamental understanding of how these cells function, it is now conceivable to build a synthetic electrogenic cell to power medical implants or to genetically engineer a more-efficient biological cell for the same uses," LaVan says. He states that the team has its eye on applying the technology to retinal prostheses once actualized.

But in order to actualize the technology, the team must first determine which design approach it will take to physically develop the cells: natural or synthetic. To engineer a synthetic version of the cells, the researchers envision a silicon or silica construction enhanced with functional coatings. However, they predict that a significant design challenge would arise in engineering a synthetic analog of the ion pump.

And yet a natural cell design would have its share of pros and cons as well. LaVan notes that one drawback would be that using proteins would incite an immune response. However, the advantages could be numerous. "The appealing feature is that you use a natural-occurring energy source that is in the body--something like glucose or fatty acids--and convert that into electricity using a biological system," explains LaVan. "The natural constraints on the biological system mean that it is inherently better suited for operation inside the body; as examples, there are no hazardous materials to worry about and no waste heat to deal with."

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